A battery separator is used to separate the positive and negative electrodes of a battery, for example, in a secondary lithium battery. A battery separator is typically microporous to allow ionic current with least possible resistance while preventing the electrodes from direct contact resulting in an internal short.
In general, a battery separator is sandwiched between the positive electrode and the negative electrode of a secondary lithium battery. It is important for a battery separator to remain in its proper position because even a minute displacement may cause a short in the battery. Currently, other than as described in US Publication US2009/0081535 A1, published Mar. 26, 2009, there are no prevailing techniques to determine the position of a separator in a battery to prevent the introduction of flawed batteries, i.e. those batteries in which the battery separator (or electrode) was displaced during the manufacturing process, into the consumer market.
Microporous polymer membranes are known, can be made by various processes, and the process by which the membrane is made may have an impact upon the membrane's physical attributes. See, for example, Kesting, Robert E., Synthetic Polymeric Membranes, A Structural Perspective, Second Edition, John Wiley & Sons, New York, N.Y., (1985). Three different known processes for making microporous polymer membranes include: the dry-stretch process (also known as the CELGARD process), the wet process, and the particle stretch process.
The dry-stretch process (the CELGARD process) refers to a process where pore formation results from stretching a nonporous, semicrystalline, extruded polymer precursor in the machine direction (MD stretch). See, for example, Kesting, Ibid. pages 290-297, incorporated herein by reference. Such a dry-stretch process is different from the wet process and the particle stretch process. Generally, in the wet process, also known as the phase inversion process, the extraction process, or the TIPS process, the polymeric raw material is mixed with a processing oil (sometimes referred to as a plasticizer), this mixture is extruded, and pores are then formed when the processing oil is removed (these films may be stretched before or after the removal of the oil). See, for example, Kesting, Ibid. pages 237-286, incorporated herein by reference.
Generally, in the particle stretch process, the polymeric raw material is mixed with a pore formation particulate, this mixture is extruded, and pores are formed during stretching when the interfaces between the polymer and the particulate fracture due to the stretching forces. See, for example, U.S. Pat. Nos. 6,057,061 and 6,080,507, each incorporated herein by reference.
Moreover, the membranes arising from these different formation processes are usually physically different and the process by which each is made typically distinguishes one membrane from the other. For example, dry-stretch process membranes may have slit shaped pores due to the stretching of the precursor in the machine direction (MD stretch). Wet process membranes tend to have rounder pores and a lacelike appearance due to the oil or plasticizer and the stretching of the precursor in the machine direction (MD stretch) and in the transverse machine direction or transverse direction (TD stretch). Particle stretch process membranes, on the other hand, may have oval shaped pores as the particulate and machine direction stretching (MD stretch) tend to form the pores. Accordingly, each membrane may be distinguished from the other by its method of manufacture.
While membranes made by the dry-stretch process have met with excellent commercial success, such as a variety of Celgard® dry-stretch porous membranes sold by Celgard, LLC of Charlotte, N.C., including flat sheet membranes, battery separators, hollow fibers, and the like, there is a need to improve, modify or enhance at least selected physical attributes thereof, so that they may be used in a wider spectrum of applications, may perform better for particular purposes, or the like.
A modified dry-stretch process (modified CELGARD process) involving the formation of unique round shaped pores by, for example, stretching a nonporous, semicrystalline, extruded polymer precursor in the machine direction (MD stretch), followed by stretching in the transverse direction (TD stretch) with machine direction relax (MD relax) is disclosed in US Published Application US2007/0196638 A1, published Aug. 23, 2007, and incorporated by reference herein.
Despite the research efforts in developing battery separators, there may still be a need for an improved battery separator, such as a battery separator which is x-ray sensitive or readily detectable when inserted or embedded in a battery, cell, stack, jellyroll, can, or the like, to determine its position within the battery, cell, stack, jellyroll, can, or the like, or relative to the electrodes, which is relatively easy to manufacture, is low cost, meets performance requirements, meets product specifications, or the like. Furthermore, there may still be a need for a method for detecting the position of a separator in a battery, cell, stack, jellyroll, can, or the like to determine its position within the battery, cell, stack, jellyroll, can, or the like, or relative to the electrodes, which is relatively easy and cost effective, a method for manufacturing such a separator which is relatively simple and cost effective, a method for using such a separator which is relatively simple and cost effective, or the like.